The present invention relates to a pigmented ink, in particular, an ink suitable for an ink-jet printing method comprising jet of ink droplets from a print head. More particularly, it relates to a pigmented ink excellent in the dispersibility and dispersion stability of pigment particles, storage stability, etc.
Since dye-dissolved inks conventionally used for ink-jet recording have defects such as low lightfastness and waterfastness, pigmented inks excellent in lightfastness and waterfastness have come to be used in some cases. The pigmented inks have to contain dispersed water-insoluble pigment particles and have to be stably storable. For example, Japanese Patent Laid-Open Nos. 62-116678, 1-301760, 2-255875, 4-334870 and 8-209048 disclose water-based pigmented inks.
However, in these water-based pigmented inks, pigment fine particles with an average particle diameter of 100 nm or less are very difficult to use which have come to be used with a recent increase in the print quality of ink-jet printers. Even in some of the above references in which employment of such fine particles is taken into consideration, there are not described the re-dispersibility of pigment particles and the improvement of the print quality of pigmented ink. The smaller the diameter of pigment particles, the larger the specific surface area thereof and the much larger the number thereof per a unit volume. Therefore, the degree of the interaction between the pigment and a dispersant greatly affects the dispersibility and dispersion stability of the pigment.